Driving waveforms for switchable light-collimating layer including bistable electrophoretic fluid

ABSTRACT

Methods and controllers for driving a light-collimating film including elongated chambers of bistable electrophoretic fluids. The light-collimating films are suitable to control the amount and/or direction of light incident to a transmissive substrate. Such films may be integrated into devices, such as LCD displays, to provide a zone of privacy for a user viewing the LCD display. Because the light-collimating film is switchable, it allows a user to alter the collimation of the emitted light on demand. Because the films are bistable, they do not require additional power after they have been switched to a display state.

RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationNo. 62/717,531, filed Aug. 10, 2018. The entire contents of any patent,published application, or other published work referenced herein isincorporated by reference.

BACKGROUND OF INVENTION

This invention relates to switchable light-collimating films that can beused, e.g., to control the directionality of incident light passingthrough a transparent or translucent substrate. Passive films that havethis ability have been commercially-available for some time, and arewidely-sold for use as “privacy filters” for computer monitors. See,e.g., offerings from 3M Corporation, St. Paul, Minn., as well as variousUS patents, such as U.S. Pat. No. 8,213,082. Typically, a privacy filteris applied to the front surface of a video display when the user wantsto limit the images on the display to a “privacy cone” that is onlyviewable by the user. Privacy films typically employ microfabricatedchannels of plastic that are backfilled with materials having adifferent index of refraction from the plastic substrate. The interfacebetween materials creates a refractive surface and only light that isoriented in the correct direction will pass through the filter, whileother incident light that is oriented in the incorrect direction will beback-reflected and/or absorbed. This same technology can also be used asa window treatment to modify the directionality of, e.g., sunlightpassing through an exterior window.

Several groups have attempted to make an active media that can beswitched between a privacy and non-privacy state. For example, U.S.Patent Publication 2016/0179231 (231 application) describes anelectroactive privacy layer that can be used in conjunction with adisplay device. The '231 application teaches to use anelectrically-anisotropic material, such as a dielectric polymer. When anelectric field is applied, the anisotropic material is aligned with thefield, collimating the light and providing a zone of privacy for theuser. However, it is necessary to provide a constant electric potentialto the privacy layer to keep the material aligned to maintain theprivacy state. Because the privacy device requires a constant electricfield to maintain the privacy state, the device consumes additionalenergy beyond the typical energy needed for the monitor. When used witha battery-powered device, e.g., a laptop computer, the additional energyrequired to power the privacy layer will shorten the operating time ofthe battery. PCT Publication WO2013/048846 also describes an alternateswitchable privacy film that also employs anisotropic particles that areheld in an aligned position with an electric field. Similar to the '231application, devices of the '846 publication also require constantenergy to be supplied in the privacy state.

Other active switching privacy devices have been described that rely onthe movement of blocking particles within a channel as opposed toalignment of anisotropic particles. For example, US Patent PublicationNo. 2016/0011441 (441 application) describes an electrically switchableelectrochromic material that is disposed in microstructured ribs runningthe length of a privacy layer. In the '441 application, the absorptionspectrum of the electrochromic material is changed when an electriccurrent is supplied to the electrochromic material. While the actualswitching process requires a fair amount of energy (˜5 minutes of DCcurrent), the privacy layer of the '441 application is able to maintainits state for some time once the transformation is complete. Anotheralternative is described in US Patent Publication No. 2017/0097554,whereby long light control channels are formed between transparentconductive films and the channels are filled with electrophoreticmembers including transmissive dispersants and light-shieldingparticles. The electrophoretic members can be toggled between a narrowviewing field mode and a wide viewing field mode by using a set of threeshaping electrodes to control the dispersion of the light-shieldingparticles in the air gap. Manufacture of the shaping electrodes can betechnically challenging (and expensive) because of the need to create somany closely-spaced, individually-addressable electrodes.

SUMMARY OF INVENTION

Despite the availability of switchable privacy filters, e.g., usinganisotropic particle alignment, there is still a need for inexpensiveprivacy films that are not power-hungry. Accordingly, this inventiondescribes voltage waveforms for driving a light-collimating film thatincludes a plurality of elongated chambers of bistable electrophoreticfluids including light-scattering pigments. With a suitable arrangementof elongated chambers, the films can provide a 2× narrowing (or more) ofviewing angle to light passing through the film. Importantly, becausethe light-collimating films include bistable electrophoretic fluids, thelight collimating films are stable for long periods of time in the wideor narrow states, and only require energy to change from one state tothe other. Additionally, because the bistable electrophoretic fluid ispartitioned into a plurality of elongated chambers, the electrophoreticmaterials are less susceptible to settling when the samelight-collimating film is applied in different orientations with respectto gravity. Additionally, the transition speed between wide and narrowstates is improved, and the overall effect is more consistent across adevice when the bistable electrophoretic fluid is partitioned into manyelongated chambers.

Furthermore, because the light-collimating film includes a plurality ofsmall chambers, it is easy to cut the film to the desired shape/sizeafter fabrication without losing large amounts of electrophoretic fluid.This allows the same equipment to be used to create both large- andsmall-area light-collimating films. For example, a square meter sectionsheet of light-collimating film or a roll of light-collimating film canbe cut into chips of desirable size without significant loss ofelectrophoretic fluid. While some chambers will be opened during thecutting process, each chamber holds only a small amount of fluid, so theoverall loss is small. In some cases, hundreds of small sheets (e.g.,for mobile phones) can be cut from a single section sheet or roll. Insome embodiments, the elongated chambers can be fabricated with apre-determined pattern such that sheet cutting results in no loss ofelectrophoretic fluid.

Thus, in one aspect, the invention includes a method for driving aswitchable light-collimating film, wherein the switchablelight-collimating film includes a first light-transmissive electrodelayer, a collimating layer having a thickness of at least 20 μmcomprising a plurality of elongated chambers, and a secondlight-transmissive electrode layer, wherein the first and second lighttransmissive layers are disposed on either side of the collimatinglayer. Each elongated chamber has an opening and a bistableelectrophoretic fluid comprising pigment particles is disposed in eachelongated chamber. The elongated chambers are sealed with a sealinglayer that seals the bistable electrophoretic fluid within by spanningthe opening of the elongated chamber. The method includes applying atime varying voltage between the first and the second light-transmissiveelectrode layers.

Typically, the switchable light-collimating film typically has athickness of less than 500 μm, and the height of the elongated chambersis equal to or less than the thickness of the collimating layer.Typically, the elongated chambers are between 5 μm and 150 μm in width,and between 200 μm and 5 mm in length. For example the elongatedchambers can be between 5 μm and 50 μm in width, and between 50 μm and 5mm in length.

The switchable light-collimating film is typically made from a polymer,for example a polymer made from acrylate monomers, urethane monomers,styrene monomers, epoxide monomers, silane monomers, thio-ene monomers,thio-yne monomers, or vinyl ether monomers. The first or secondlight-transmissive electrode layers may be made from indium-tin-oxide.

The bistable electrophoretic fluid typically includespolymer-functionalized pigment particles and free polymer in a non-polarsolvent. Often, the pigment is functionalized with a polyacrylate,polystyrene, polynaphthalene, or polydimethylsiloxane. The free polymermay include polyisobutylene or copolymers including ethylene, propylene,or styrene monomers, and the sealing layer may include a water solublepolymer or water dispersible polymer, such as naturally-occurringwater-soluble polymers, such as cellulose or gelatin, or syntheticpolymers, such as polyacrylate, a polyvinyl alcohol, a polyethylene, apoly(vinyl) acetate, a poly(vinyl) pyrrolidone, a polyurethane, or acopolymer thereof.

In an embodiment, the time varying voltage includes a first voltage fora first time that has a first polarity and a second voltage for a secondtime that has a second polarity. In some embodiments, the first voltagehas a first magnitude, the second voltage has a second magnitude, andthe first and second magnitudes are not equal. In some embodiments, thefirst voltage has a first magnitude, the second voltage has a secondmagnitude, and the first and second magnitudes are not equal. In someembodiments, the product of the first voltage and the first time is afirst impulse, the product of the second voltage and the second time isa second impulse, and the first impulse and the second impulse are notequal in magnitude. In some embodiments, the product of the firstvoltage and the first time is a first impulse, the product of the secondvoltage and the second time is a second impulse, and the first impulseand the second impulse are equal in magnitude. In some embodiments, thetime varying voltage further includes the first voltage for a third timethat has the first polarity and the second voltage for a fourth timethat has the second polarity. In some embodiments, the time varyingvoltage is between 5 V and 150 V in magnitude, e.g., between 80 V and120 V.

In an embodiment, the elongated chambers are arranged in rows andcolumns when the collimating layer is viewed from above, wherein thelonger dimension of the elongated chambers run along rows, and whereinthe rows are separated from each other by at least three times the widthof the elongated chambers. Often, the elongated chambers are arranged inrows and columns when the collimating layer is viewed from above, andthe adjacent elongated chambers within the same row are separated by agap of less than 30 μm. In some embodiments, the gaps between adjacentelongated chambers in the first row are offset horizontally from thegaps between adjacent elongated chambers in the second row. In someembodiments, the symmetry of the elongated chambers is disrupted byaltering the length of the elongated chambers, the width of theelongated chambers, the pitch of the elongated chambers, or the width orplacement of the gap between elongated chambers.

In another aspect the invention includes a display having a lightsource, a switchable light-collimating film, an active matrix of thinfilm transistors, a liquid crystal layer, a color filter array, avoltage source, and a controller. The switchable light-collimating filmincludes a first light-transmissive electrode layer, a collimating layerhaving a thickness of at least 20 μm comprising a plurality of elongatedchambers, and a second light-transmissive electrode layer, wherein thefirst and second light transmissive layers are disposed on either sideof the collimating layer. The elongated chambers hold a bistableelectrophoretic fluid comprising pigment particles and the elongatedchambers are sealed with a sealing layer that spans the opening of theelongated chamber. The controller provides a voltage impulse between thefirst and second light-transmissive electrode layers, wherein thevoltage impulse causes a change in the effective transmission angle ofthe display.

In some embodiments, the light-collimating film or display additionallyinclude a voltage source and a controller to provide a voltage impulsebetween the first and second light-transmissive electrode layers. Insome embodiments, the display includes a prism film disposed between thelight source and the switchable light-collimating film. In someembodiments, the display includes a diffusion layer between the prismfilm and the light source. In some embodiments, the display includes atouch screen layer.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A illustrates a first state of a switchable light-collimating filmin which electrophoretic particles are distributed throughout thechambers of a collimating layer. The electrophoretic particles arestable in this state without the application of power.

FIG. 1B illustrates a second state of a switchable light-collimatingfilm in which electrophoretic particles are driven toward a firstlight-transmissive electrode with the application of an electricalpotential.

FIG. 1C illustrates a third state of a switchable light-collimating filmin which electrophoretic particles are collected in proximity to thefirst light-transmissive electrode. The particles are stable in thisposition even after the electric potential has been removed.

FIG. 1D illustrates a return to a state in which the electrophoreticparticles are distributed throughout the chambers of a collimatinglayer.

FIG. 1E illustrates a fourth state of a switchable light-collimatingfilm in which electrophoretic particles are driven toward a secondlight-transmissive electrode with the application of an electricalpotential having an opposite polarity of FIG. 1B.

FIG. 1F illustrates a fifth state of a switchable light-collimating filmin which electrophoretic particles are collected in proximity to thesecond light-transmissive electrode. The particles are stable in thisposition even after the electric potential has been removed.

FIG. 2A illustrates that when electrophoretic particles are distributedthroughout chambers of a collimating layer, rays of light emitted from asource are limited to an angle θ₁.

FIG. 2B illustrates that when electrophoretic particles are collectedagainst a light-transmissive electrode closest to the light source, raysof light emitted from the source at an angle θ₂, wherein θ₂>>θ₁.

FIG. 2C illustrates that when electrophoretic particles are collectedagainst a light-transmissive electrode furthest from the light source,rays of light emitted from the source at an angle θ₃, wherein θ₃>>θ₁. Itis observed that there is minimal light loss due to the presence of thepigment particles at the emissive side of the light-collimating film.

FIG. 3A illustrates simple waveforms that can be used to switch alight-collimating film between a first state and a second state.

FIG. 3B illustrates complex waveforms that can be used to switch alight-collimating film between a first state and a second state.

FIG. 4 illustrates the operative layers of a liquid crystal displayassembly including a switchable light-collimating film. The layers arenot to scale.

FIG. 5A illustrates the effective viewing angle, φ, of an LCD displaystack including a light-collimating layer when the electrophoreticparticles are dispersed in the elongated chambers. Note that Viewer #2does not “see” much of the light from the LCD display stack.

FIG. 5B illustrates the effective viewing angle, φ, of an LCD displaystack including a light-collimating layer when the electrophoreticparticles are packed at the bottom (or top) of the elongated chambers.

FIG. 5C illustrates the intensity of light as a function of effectiveviewing angle when an LCD display stack including a light-collimatinglayer is in the first state (see FIG. 5A) and the second state (see FIG.5B).

FIG. 6 illustrates an embodiment of a switchable light-collimating filmdisposed on a lower substrate. The switchable light-collimating filmadditionally includes an edge seal. The exploded view details thesealing layer atop the elongated chamber filled with bistableelectrophoretic fluid.

FIG. 7 illustrates a switchable light-collimating film that has anoptically clear adhesive and a release sheet on one side. Such films maybe used, for example, to provide collimating features on existingsurfaces, such as a glass windows.

FIG. 8 illustrates a roll-to-roll process that can be used for forming acollimating layer with a plurality of elongated chambers andsubsequently filling the elongated chambers with a bistableelectrophoretic fluid and sealing the filled elongated chambers.

FIGS. 9A and 9B illustrate a simplified embossing process.

FIG. 10 details a method for forming an embossing tool to createcollimating layers of the invention.

FIG. 11 details a method for forming a shim to be used in an embossingtool.

FIG. 12 details an alternate method for forming a shim to be used in anembossing tool.

FIG. 13 is a top view of an embodiment of a switchable light-collimatingfilm in which elongated chambers are arranged in a row-column format.

FIG. 14 is a top view of an embodiment of a switchable light-collimatingfilm in which elongated chambers are arranged in a row-column format.

DETAILED DESCRIPTION

As indicated above, the present invention provides driving waveforms fora light-collimating film that includes elongated chambers of bistableelectrophoretic fluids. Such films can be used, on their own, to controlthe amount and/or direction of light incident to a transmissivesubstrate. Such films can also be integrated into devices, such as anLCD display, to provide useful features such as a zone of privacy for auser viewing the LCD display. Using the disclosed waveforms, thecollimation of the emitted light can be altered on demand. Additionally,because the electrophoretic medium is bistable, the collimation statewill be stable for some time, e.g., minutes, e.g., hours, e.g., days,e.g., months, without the need to provide additional energy to thelight-collimating film.

The systems described herein can be fabricated inexpensively usingroll-to-roll processing. Accordingly, it is feasible to produce largesheets of switchable light-collimating film that can be incorporatedinto devices during other assembly processes, such as the fabrication ofan LCD display. Such films may include an auxiliary optically-clearadhesive layer and a release sheet, thereby allowing thelight-collimating film to be shipped and distributed as a finishedproduct. The light-collimating film may also be used for after-marketlight control, for example for conference room windows, exterior windowsin buildings, and sunroofs and skylights. The first and secondelectrodes of the light-collimating films are easily accessed, using forexample laser cutting, so it is quite easy to connect a voltage sourceand controller to the electrodes.

An electrophoretic display normally comprises a layer of electrophoreticmaterial and at least two other layers disposed on opposed sides of theelectrophoretic material, one of these two layers being an electrodelayer. In most such displays both the layers are electrode layers, andone or both of the electrode layers are patterned to define the pixelsof the display. For example, one electrode layer may be patterned intoelongate row electrodes and the other into elongate column electrodesrunning at right angles to the row electrodes, the pixels being definedby the intersections of the row and column electrodes. Alternatively,and more commonly, one electrode layer has the form of a singlecontinuous electrode and the other electrode layer is patterned into amatrix of pixel electrodes, each of which defines one pixel of thedisplay. In some embodiments, two light-transmissive electrode layersare used, thereby allowing light to pass through the electrophoreticdisplay.

The terms “bistable” and “bistability” are used herein in theirconventional meaning in the art to refer to displays comprising displayelements having first and second display states differing in at leastone optical property, and such that after any given element has beendriven, by means of an addressing pulse of finite duration, to assumeeither its first or second display state, after the addressing pulse hasterminated, that state will persist for at least several times, forexample at least four times, the minimum duration of the addressingpulse required to change the state of the display element. It is shownin U.S. Pat. No. 7,170,670 that some particle-based electrophoreticdisplays capable of gray scale are stable not only in their extremeblack and white states but also in their intermediate gray states, andthe same is true of some other types of electro-optic displays. Thistype of display is properly called “multi-stable” rather than bistable,although for convenience the term “bistable” may be used herein to coverboth bistable and multi-stable displays.

The general function of a switchable light-collimating film (10) isshown in FIGS. 1A-1F. The film (10) includes first (12) and second (14)light-transmissive electrode layers. Typically, each electrode layer isassociated with a first substrate (16) and a second substrate (18),respectively. The first (16) and second (18) substrates may be alight-transmissive polymer (e.g., a film or resin) or glass. In theinstance that the film (10) is produced with roll-to-roll processing,the first (16) and second (18) substrates are flexible. Thelight-transmissive electrodes and the substrates may also be integratedinto a single layer, for example a PET-ITO film, PEDOT, or anotherlight-transmissive polymer that is doped with a conductive material(e.g., graphene, nanotubes, metal flakes, conductive metal oxideparticles, or metal fibers) and/or doped with conductive monomers orpolymers and/or doped with ionic materials, such as salts.

The light-collimating layer (21) comprises a light-transmissive polymer(20) that has been processed to produce a plurality of elongatedchambers (22) to hold a bistable electrophoretic fluid (24) thatincludes electrophoretic particles (26). In an embodiment, the bistableelectrophoretic fluid (24) includes a hydrocarbon solvent and theelectrophoretic particles (26) comprise carbon black (optionallyfunctionalized as discussed below). The light-collimating layer is atleast 20 μm thick (i.e., the distance between the first (12) and second(14) light-transmissive electrode layers). The light-collimating layercan be thicker than 20 μm, for example thicker than 30 μm, for examplethicker than 50 μm, for example thicker than 70 μm, for example thickerthan 100 μm, for example thicker than 150 μm, for example thicker than200 μm. The fabrication of the elongated chambers, e.g., by embossing athermoplastic, is described in greater detail below. After, or duringthe process of filing the elongated chambers (22), the elongatedchambers (22) are sealed with a sealing layer (28), which may be, forexample, a hydrophilic polymer that is incompatible with the bistableelectrophoretic fluid (24).

In order to change the collimating properties of the film (10) the first(12) and second (14) light-transmissive electrode layers may be coupledto a source (30) of an electrical potential. The source may be, e.g., abattery, a power supply, a photovoltaic, or some other source ofelectrical potential. A controller (not shown) is used to supply a timevarying voltage between the first (12) and the second (14)light-transmissive electrode layers The first (12) and second (14)light-transmissive electrode layers may be coupled to the source (30)via electrodes, wires, or traces (31). In some embodiments, the traces(31) may be interrupted with a switch (32) which may be, e.g., atransistor switch. The electrical potential between the first (12) andsecond (14) light-transmissive electrode layers is typically at leastone volt, for example at least two volts, for example at least fivevolts, for example at least ten volts, for example at least 15 volts,for example at least 18 volts, for example at least 25 volts, forexample at least 30 volts, for example at least 30 volts, for example atleast 50 volts. In some embodiments, the controller supplies a timevarying voltage between 5 volts and 150 volts in magnitude, e.g.,between 80 volts and 120 volts in magnitude.

Because the bistable electrophoretic fluid (24) is bistable, theelectrophoretic particles (26) will maintain their distribution withoutapplication of an electric field. This feature is well-described in EInk Corporation patents listed herein, but mostly results from having aspecific mixture of distributed polymers (e.g., polyisobutylene orpolylaurylmathacrylate) in the bistable electrophoretic fluid (24) sothat the electrophoretic particles (26) are stabilized via depletionflocculation. Accordingly, in a first state, illustrated in FIG. 1A, theelectrophoretic particles (26) are stable in a dispersed state, despiteno electrical potential being applied between the first (12) and second(14) light-transmissive electrode layers. With the application of asuitable electric potential, e.g., as illustrated in FIG. 1B, theelectrophoretic particles (26) move toward the suitably biased electrodelayer, creating a light-transmission gradient along the height of theelongated chambers (22). Once the electrophoretic particles (26) aredriven to the desired electrode layer, the source (30) can be decoupledfrom the electrode layers, turning off the electric potential. However,because of the bistability of the bistable electrophoretic fluid (24),the electrophoretic particles (26) will remain in the second state of along period of time, e.g., minutes, e.g., hours, e.g., days, as shown inFIG. 1C.

The state of the light-collimating film (10) can be reversed by drivingthe collected electrophoretic particles (26) away from the electrodewith a reverse polarity voltage (not shown) to achieve FIG. 1D. Uponreturning to the initial state (equivalent to 1A), only (roughly)collimated light will be able to pass the light-collimating film, asdescribed in greater detail below. The state of FIG. 1D is also stable.The electrophoretic particles (26) can be driven past this distributedstate and toward the second light-transmissive electrode (14) withapplication of the reverse-polarity voltage from FIG. 1B, as shown inFIG. 1E. As a result, the electrophoretic particles (26) will collectadjacent the second light-transmissive electrode (14) which also resultsin a wide-viewing angle, as discussed below. The wide-angle transmissivestate shown in FIG. 1F is also bistable, that is, no power is requiredto maintain this state. Because the states of FIG. 1C and FIG. 1F bothresult in wide-angle transmission, it is possible to toggle between thestates shown in FIGS. 1A, 1C, 1D, and 1F while maintaining an overall DCbalance on the drive electronics. DC balancing the drive electronicsreduces charge build-up and extends the life of the system components.

The internal phase of the electrophoretic medium includes chargedpigment particles in a suspending fluid. The fluids used in the variabletransmission media of the present invention will typically be of lowdielectric constant (preferably less than 10 and desirably less than 3).Especially preferred solvents include aliphatic hydrocarbons such asheptane, octane, and petroleum distillates such as Isopar® (Exxon Mobil)or Isane® (Total); terpenes such as limonene, e.g., 1-limonene; andaromatic hydrocarbons such as toluene. A particularly preferred solventis limonene, since it combines a low dielectric constant (2.3) with arelatively high refractive index (1.47). The index of refraction of theinternal phase may be modified with the addition of index matchingagents such as Cargille® index matching fluids available fromCargille-Sacher Laboratories Inc. (Cedar Grove, N.J.). In encapsulatedmedia of the present invention, it is preferred that the refractiveindex of the dispersion of particles match as closely as possible thatof the encapsulating material to reduce haze. This index-matching isbest achieved (when employing commonly-available polymeric encapsulants)when the refractive index of the solvent is close to that of theencapsulant. In most instances, it is beneficial to have an internalphase with an index of refraction between 1.51 and 1.57 at 550 nm,preferably about 1.54 at 550 nm.

Charged pigment particles may be of a variety of colors andcompositions. Additionally, the charged pigment particles may befunctionalized with surface polymers to improve state stability. Suchpigments are described in U.S. Patent Publication No. 2016/0085132,which is incorporated by reference in its entirety. For example, if thecharged particles are of a white color, they may be formed from aninorganic pigment such as TiO2, ZrO2, ZnO, Al2O3, Sb2O3, BaSO4, PbSO4 orthe like. They may also be polymer particles with a high refractiveindex (>1.5) and of a certain size (>100 nm) to exhibit a white color,or composite particles engineered to have a desired index of refraction.Black charged particles, they may be formed from CI pigment black 26 or28 or the like (e.g., manganese ferrite black spinel or copper chromiteblack spinel) or carbon black. Other colors (non-white and non-black)may be formed from organic pigments such as CI pigment PR 254, PR122,PR149, PG36, PG58, PG7, PB28, PB15:3, PY83, PY138, PY150, PY155 or PY20.Other examples include Clariant Hostaperm Red D3G 70-EDS, Hostaperm PinkE-EDS, PV fast red D3G, Hostaperm red D3G 70, Hostaperm Blue B2G-EDS,Hostaperm Yellow H4G-EDS, Novoperm Yellow HR-70-EDS, Hostaperm GreenGNX, BASF Irgazine red L 3630, Cinquasia Red L 4100 HD, and Irgazin RedL 3660 HD; Sun Chemical phthalocyanine blue, phthalocyanine green,diarylide yellow or diarylide AAOT yellow. Color particles can also beformed from inorganic pigments, such as CI pigment blue 28, CI pigmentgreen 50, CI pigment yellow 227, and the like. The surface of thecharged particles may be modified by known techniques based on thecharge polarity and charge level of the particles required, as describedin U.S. Pat. Nos. 6,822,782, 7,002,728, 9,366,935, and 9,372,380 as wellas US Publication No. 2014-0011913, the contents of all of which areincorporated herein by reference in their entirety.

The particles may exhibit a native charge, or may be charged explicitlyusing a charge control agent, or may acquire a charge when suspended ina solvent or solvent mixture. Suitable charge control agents are wellknown in the art; they may be polymeric or non-polymeric in nature ormay be ionic or non-ionic. Examples of charge control agent may include,but are not limited to, Solsperse 17000 (active polymeric dispersant),Solsperse 9000 (active polymeric dispersant), OLOA 11000 (succinimideashless dispersant), Unithox 750 (ethoxylates), Span 85 (sorbitantrioleate), Petronate L (sodium sulfonate), Alcolec LV30 (soy lecithin),Petrostep B100 (petroleum sulfonate) or B70 (barium sulfonate), AerosolOT, polyisobutylene derivatives or poly(ethylene co-butylene)derivatives, and the like. In addition to the suspending fluid andcharged pigment particles, internal phases may include stabilizers,surfactants and charge control agents. A stabilizing material may beadsorbed on the charged pigment particles when they are dispersed in thesolvent. This stabilizing material keeps the particles separated fromone another so that the variable transmission medium is substantiallynon-transmissive when the particles are in their dispersed state. As isknown in the art, dispersing charged particles (typically a carbonblack, as described above) in a solvent of low dielectric constant maybe assisted by the use of a surfactant. Such a surfactant typicallycomprises a polar “head group” and a non-polar “tail group” that iscompatible with or soluble in the solvent. In the present invention, itis preferred that the non-polar tail group be a saturated or unsaturatedhydrocarbon moiety, or another group that is soluble in hydrocarbonsolvents, such as for example a poly(dialkylsiloxane). The polar groupmay be any polar organic functionality, including ionic materials suchas ammonium, sulfonate or phosphonate salts, or acidic or basic groups.Particularly preferred head groups are carboxylic acid or carboxylategroups. Stabilizers suitable for use with the invention includepolyisobutylene and polystyrene. In some embodiments, dispersants, suchas polyisobutylene succinimide and/or sorbitan trioleate, and/or2-hexyldecanoic acid are added.

The electrophoretic media of the present invention will typicallycontain a charge control agent (CCA), and may contain a charge director.These electrophoretic media components typically comprise low molecularweight surfactants, polymeric agents, or blends of one or morecomponents and serve to stabilize or otherwise modify the sign and/ormagnitude of the charge on the electrophoretic particles. The CCA istypically a molecule comprising ionic or other polar groupings,hereinafter referred to as head groups. At least one of the positive ornegative ionic head groups is preferably attached to a non-polar chain(typically a hydrocarbon chain) that is hereinafter referred to as atail group. It is thought that the CCA forms reverse micelles in theinternal phase and that it is a small population of charged reversemicelles that leads to electrical conductivity in the very non-polarfluids typically used as electrophoretic fluids.

Reverse micelles comprise a highly polar core (that typically containswater) that may vary in size from 1 nm to tens of nanometers (and mayhave spherical, cylindrical, or other geometry) surrounded by thenon-polar tail groups of the CCA molecule. Reverse micelles have beenextensively studied, especially in ternary mixtures such asoil/water/surfactant mixtures. An example is the iso-octane/water/AOTmixture described, for example, in Fayer et al., J. Chem. Phys., 131,14704 (2009). In electrophoretic media, three phases may typically bedistinguished: a solid particle having a surface, a highly polar phasethat is distributed in the form of extremely small droplets (reversemicelles), and a continuous phase that comprises the fluid. Both thecharged particles and the charged reverse micelles may move through thefluid upon application of an electric field, and thus there are twoparallel pathways for electrical conduction through the fluid (whichtypically has a vanishingly small electrical conductivity itself).

The polar core of the CCA is thought to affect the charge on surfaces byadsorption onto the surfaces. In an electrophoretic display, suchadsorption may be onto the surfaces of the electrophoretic particles orthe interior walls of a microcapsule (or other solid phase, such as thewalls of a microcell) to form structures similar to reverse micelles,these structures hereinafter being referred to as hemi-micelles. Whenone ion of an ion pair is attached more strongly to the surface than theother (for example, by covalent bonding), ion exchange betweenhemi-micelles and unbound reverse micelles can lead to charge separationin which the more strongly bound ion remains associated with theparticle and the less strongly bound ion becomes incorporated into thecore of a free reverse micelle.

It is also possible that the ionic materials forming the head group ofthe CCA may induce ion-pair formation at the electrophoretic particle(or other) surface. Thus the CCA may perform two basic functions:charge-generation at the surface and charge-separation from the surface.The charge-generation may result from an acid-base or an ion-exchangereaction between some moieties present in the CCA molecule or otherwiseincorporated into the reverse micelle core or fluid, and the particlesurface. Thus, useful CCA materials are those which are capable ofparticipating in such a reaction, or any other charging reaction asknown in the art. The CCA molecules may additionally act as receptors ofthe photo-excitons produced by the electrophoretic particles when theparticles are irradiated with light.

Non-limiting classes of charge control agents which are useful in themedia of the present invention include organic sulfates or sulfonates,metal soaps, block or comb copolymers, organic amides, organiczwitterions, and organic phosphates and phosphonates. Useful organicsulfates and sulfonates include, but are not limited to, sodiumbis(2-ethylhexyl) sulfosuccinate, calcium dodecylbenzenesulfonate,calcium petroleum sulfonate, neutral or basic barium dinonylnaphthalenesulfonate, neutral or basic calcium dinonylnaphthalene sulfonate,dodecylbenzenesulfonic acid sodium salt, and ammonium lauryl sulfate.Useful metal soaps include, but are not limited to, basic or neutralbarium petronate, calcium petronate, cobalt, calcium, copper, manganese,magnesium, nickel, zinc, aluminum and iron salts of carboxylic acidssuch as naphthenic, octanoic, oleic, palmitic, stearic, and myristicacids and the like. Useful block or comb copolymers include, but are notlimited to, AB diblock copolymers of (A) polymers of2-(N,N-dimethylamino)ethyl methacrylate quaternized with methylp-toluenesulfonate and (B) poly(2-ethylhexyl methacrylate), and combgraft copolymers with oil soluble tails of poly(12-hydroxystearic acid)and having a molecular weight of about 1800, pendant on an oil-solubleanchor group of poly(methyl methacrylate-methacrylic acid). Usefulorganic amides/amines include, but are not limited to, polyisobutylenesuccinimides such as OLOA 371 or 1200 (available from Chevron OroniteCompany LLC, Houston, Tex.), or Solsperse 17000 (available fromLubrizol, Wickliffe, Ohio: Solsperse is a Registered Trade Mark), andN-vinylpyrrolidone polymers. Useful organic zwitterions include, but arenot limited to, lecithin. Useful organic phosphates and phosphonatesinclude, but are not limited to, the sodium salts of phosphated mono-and di-glycerides with saturated and unsaturated acid substituents.Useful tail groups for CCA include polymers of olefins such aspoly(isobutylene) of molecular weight in the range of 200-10,000. Thehead groups may be sulfonic, phosphoric or carboxylic acids or amides,or alternatively amino groups such as primary, secondary, tertiary orquaternary ammonium groups.

Charge adjuvants used in the media of the present invention may bias thecharge on electrophoretic particle surfaces, as described in more detailbelow. Such charge adjuvants may be Bronsted or Lewis acids or bases.

Particle dispersion stabilizers may be added to prevent particleflocculation or attachment to the capsule or other walls or surfaces.For the typical high resistivity liquids used as fluids inelectrophoretic displays, non-aqueous surfactants may be used. Theseinclude, but are not limited to, glycol ethers, acetylenic glycols,alkanolamides, sorbitol derivatives, alkyl amines, quaternary amines,imidazolines, dialkyl oxides, and sulfosuccinates.

As described in U.S. Pat. No. 7,170,670, the bistability ofelectrophoretic media can be improved by including in the fluid apolymer having a number average molecular weight in excess of about20,000, this polymer being essentially non-absorbing on theelectrophoretic particles; poly(isobutylene) is a preferred polymer forthis purpose.

Also, as described in for example, U.S. Pat. No. 6,693,620, a particlewith immobilized charge on its surface sets up an electrical doublelayer of opposite charge in a surrounding fluid. Ionic head groups ofthe CCA may be ion-paired with charged groups on the electrophoreticparticle surface, forming a layer of immobilized or partiallyimmobilized charged species. Outside this layer is a diffuse layercomprising charged (reverse) micelles comprising CCA molecules in thefluid. In conventional DC electrophoresis an applied electric fieldexerts a force on the fixed surface charges and an opposite force on themobile counter-charges, such that slippage occurs within the diffuselayer and the particle moves relative to the fluid. The electricpotential at the slip plane is known as the zeta potential.

A light-collimating film (10) can be used to narrow (collimate) light(33) as shown in FIGS. 2A, 2B, and 2C. In a first, narrowed state, shownin FIG. 2A, the electrophoretic particles (26) are distributedthroughout the elongated chambers (22) resulting in transmission angleθ₁ that is defined by the pitch (A) between elongated chambers (22), thewidth (W) of each elongated chamber (22), the height (H) of thelight-collimating film (10), and the distance from the source of thelight (33) to the exiting substrate (in the example of FIG. 2A,substrate (18)). As can be seen in FIG. 2A, the angle θ₁ is roughlydefined by the rays X-X′ and Y-Y′, which define the greatest angle fromnormal that light can leave the source (33) and clear both the top andthe bottom of the elongated chamber (22) with electrophoretic particles(26) distributed throughout.

In a first wide-angle state, equivalent to FIG. 1C above, theelectrophoretic particles (26) are driven to the nearer substrate (16),and a new transmission angle θ₂ is established for the rays X-X′ andY-Y′, as shown in FIG. 2B. The new transmission angle θ₂ will be muchwider than θ₁, as shown in FIG. 2B, that is, θ₂>>θ₁. Again, theeffective narrowing of the transmission angle θ₂ will be a function ofthe pitch (A) between elongated chambers (22), the width (W) of eachelongated chamber (22), and the height (H) of the light-collimating film(10).

In a second wide-angle state, equivalent to FIG. 1F above, theelectrophoretic particles (26) are driven to the substrate (16) awayfrom the light source (33), and a new transmission angle θ₃ isestablished for the rays X-X′ and Y-Y′, as shown in FIG. 2C. The newtransmission angle θ₃ will be much wider than θ₁, as shown in FIG. 2C,that is, θ₃>>θ₁. Like FIG. 2B, the effective narrowing of thetransmission angle θ₃ will be a function of the pitch (A) betweenelongated chambers (22), the width (W) of each elongated chamber (22),and the height (H) of the light-collimating film (10). Furthermore,while it would seem that a shadow may be cast by the electrophoreticparticles (26) accumulated adjacent to the second substrate (18), thisis not observed. It is surmised that there is sufficient scattered lightthrough the light-collimating film (10) to wash out this effect.

A variety of time-varying voltages (waveforms) can be applied betweenthe first and the second light-transmissive electrode layers in order tochange the state of the light-collimating film. As shown in FIG. 3A, thewaveforms can be simple D.C. voltages that are biased to cause theelectrophoretic particles (26) to move within the elongated chambers(22). For example, to move between a dispersed and a packed state, awaveform that is simple a D.C. voltage of +V₀ can be applied for sometime, t₀, (see WF₁) thereby causing the electrophoretic particles tomove toward the first electrode (12), as shown in FIG. 3A. The processcan be reversed by providing a new D.C. voltage of −V₀ for some time t₁,(see WF₂) however the initial (disperse) state of the electrophoreticparticles (26) is not completely reversed by simply applying the samemagnitude, opposite polarity, voltage for an equivalent time. Typically,the opposite polarity must be applied for a shorter time, that is t₀>t₁so that the pigment is distributed throughout the elongated chambers(22).

In practice, it is more reliable to use waveforms such as shown in FIG.3B to control the position of the electrophoretic particles (26). Inparticular, WF₃ is an alternating voltage waveform that is more negativethan positive when integrated over time, thereby resulting in theelectrophoretic particles (26) being moved toward the second electrode(14). It has been found empirically that the packing state of theelectrophoretic particles (26) is superior when WF₃ is used as comparedto a D.C. waveform, such as shown in FIG. 3A. Furthermore, regardless ofhow the packed state has been addressed, an alternating balancedwaveform such as WF₄ is far superior in redistributing theelectrophoretic particles (26) within the elongated chambers (22).

It is expected that in most configurations, light-collimating films (10)of the invention will provide at least a two-fold reduction in effectiveviewing angle (as defined by less than 50% percent relative transmissionas a function of angle from the normal) in transitioning fromwide-transmission angle (FIGS. 2B and 2C) to narrow-transmission angle(FIG. 2A). In some embodiments, the reduction in viewing area will begreater than two-fold, e.g., three-fold, e.g., four-fold. Because ofthis functionality, light-collimating film (10) may be useful whensimply applied to a pane of glass, e.g., an interior office window,whereby the transmission angle of the glass can be greatly reduced,thereby increasing privacy for the occupants of the office while stillallowing a good amount of light to transit through the window.

A light-collimating film (10) may be incorporated into a liquid crystaldisplay (LCD) stack as shown in FIG. 4. FIG. 4 is exemplary, as thereare a number of different configurations for LCD stacks. As shown inFIG. 4 a light (33), which is typically one or more light-emittingdiodes (LEDs), is directed through the display stack, including theactive layer, by a combination of a light-guide plate (34) and adiffuser plate (35). The light that leaves the diffuser plate (35),travelling in the direction of the viewer (eye at top of FIG. 4), nextencounters a light-collimating film (10), of the type described above.In the state shown in FIG. 4, the light-collimating film (10) will onlyallow light to pass to the active layer when that light is travellingwithin a narrower transmission angle (see FIGS. 2A-2C). The light thatdoes pass through the light-collimating film (10) will next proceedthrough a first polarizing film (36), an active matrix thin filmtransistor (AM-TFT) array (40) including a plurality of pixel electrodes(42). The polarized light passing through the AM-TFT (40) and pixelelectrodes (42) will then encounter a liquid crystal layer (44), wherebythe polarization of the light can be manipulated by the liquid crystalssuch that the light will be transmitted through as second polarizingfilm (37) or rejected. Specifically, the optical state of the liquidcrystal layer (44) is altered by providing an electric field between apixel electrode and the front electrode (45), as is known in the art ofLCD displays. The light that is transmitted through thelight-collimating film (10), AM-TFT (40), pixel electrodes (42), liquidcrystal layer (44), and front electrode (45) will then transmit througha color filter array (46) which will only pass the spectrum of colorthat is to be associated with the underlying pixel electrode (42).Finally, some amount of light that is of the correct color and thecorrect polarization (as determined by the liquid crystal layer) willtraverse the second polarizing film (37) and be viewed by the viewer.Various additional layers of optical adhesives (47) may be included inthe stack where needed. The stack may also include a protective coverlayer (49) which may be, e.g., glass or plastic. Additional elements,such as a capacitive touch-sensitive layer (48) or a digitizer layer(not shown) may also be added to the stack to achieve touch screencapability or writing capability, etc. Optionally, an LCD stack may alsoinclude a protective cover layer and/or a capacitive touch-sensitivelayer.

The net effect of the LCD stack including a light-collimating film (10)illustrated in FIG. 4 is that it is possible to independently controlthe effective viewing angle, (p, of light emanating from an LCD display,e.g., a computer monitor, smart phone, data terminal, or other LCDdisplay. Furthermore, because the switching medium is bistable, thedevice can remain in a “wide” or “narrow” state virtually indefinitely.In advanced embodiments, the amount of narrowing can be adjusted bycontrolling the relative amount of pigment that is driven toward theviewing side of the elongated chambers. The effective viewing angle canbe adjusted completely independent of the state of the LCD. That is, itis not necessary to power down the monitor to switch between a privacyand non-privacy mode.

The variation in effective viewing angle, φ, for an LCD display stack isillustrated in FIGS. 5A-5C. When the electrophoretic particles (26) aredispersed throughout the elongated chambers (22), rays having atransmission angle θ₁ can travel through the liquid crystal material andcolor filter material, resulting in an LCD image that is viewable withinan effective viewing angle, φ₁, as shown in FIG. 5A. As a result, Viewer#1 (the user) sees the expected image on the LCD screen. However, Viewer#2 is outside of the effective viewing angle, and cannot see what is onthe LCD screen. That is, Viewer #1 is experiencing a cone of privacy.

When the electrophoretic particles (26) are packed against one of theelectrodes, however, rays having a transmission angle θ₂ can travelthrough the liquid crystal material and color filter material, resultingin an LCD image that is viewable within an effective viewing angle, φ₂,as shown in FIG. 5B. In this configuration, Viewer #2 is able to see animage on the LCD screen. The effective viewing angle-dependent intensityof the emitted light is shown graphically in FIG. 5C. Thus, Viewer #1experiences approximately the same intensity of emitted light from theLCD screen when viewing along the normal, however Viewer #2 may see noneof the image at Viewer #1's choosing.

An exploded view of the sealing layer (28) is shown in FIG. 6. In someembodiments, the sealing layer (28) seals a top portion of the elongatedchamber (22) as shown in the exploded view in order to hold the bistableelectrophoretic fluid (24). This may be achieved by under-filling theelongated chamber (22) with bistable electrophoretic fluid (24) and thenovercoating the most full elongated chambers (22) with the sealingformulation (discussed below). In other embodiments, the sealingcomposition may be dispersed in the bistable electrophoretic fluid (24)at the time of filling, but designed with the correct hydropholicity anddensity to cause the sealing formulation to rise to the top of theelongated chamber (22) whereby it is hardened, e.g., using light, heat,or exposure to an activating chemical agent. In alternative embodiments(not shown in FIG. 6) the elongated chambers (22) may be filled to thetop and the sealing layer spread over the entirety of the top of thelight-transmissive polymer (20), thereby sealing the bistableelectrophoretic fluid (24) within the elongated chambers.

Examples of essential components in a sealing composition for thesealing layer may include, but are not limited to, thermoplastic orthermoset and their precursor thereof. Specific examples may includematerials such as monofunctional acrylates, monofunctionalmethacrylates, multifunctional acrylates, multifunctional methacrylates,polyvinyl alcohol, polyacrylic acid, cellulose, gelatin or the like.Additives such as a polymeric binder or thickener, photoinitiator,catalyst, vulcanizer, filler, colorant or surfactant may be added to thesealing composition to improve the physico-mechanical properties and thelight-collimating film.

The sealing composition may be a water soluble polymer with water as thesealing solvent. Examples of suitable water soluble polymers or watersoluble polymer precursors may include, but are not limited to,polyvinyl alcohol; polyethylene glycol, its copolymers withpolypropylene glycol, and its derivatives, such as PEG-PPG-PEG, PPG-PEG,PPG-PEG-PPG; poly(vinylpyrrolidone) and its copolymers such aspoly(vinylpyrrolidone)/vinyl acetate (PVP/VA); polysaccharides such ascellulose and its derivatives, poly(glucosamine), dextran, guar gum, andstarch; gelatin; melamine-formaldehyde; poly(acrylic acid), its saltforms, and its copolymers; poly(methacrylic acid), its salt forms, andits copolymers; poly(maleic acid), its salt forms, and its copolymers;poly(2-dimethylaminoethyl methacrylate); poly(2-ethyl-2-oxazoline);poly(2-vinylpyridine); poly(allylamine); polyacrylamide;polyethylenimine; polymethacrylamide; poly(sodium styrene sulfonate);cationic polymer functionalized with quaternary ammonium groups, such aspoly(2-methacryloxyethyltrimethylammonium bromide), poly(allylaminehydrochloride). The sealing material may also include a waterdispersible polymer with water as a formulating solvent. Examples ofsuitable polymer water dispersions may include polyurethane waterdispersion and latex water dispersion. Suitable latexes in the waterdispersion include polyacrylate, polyvinyl acetate and its copolymerssuch as ethylene vinyl acetate, and polystyrene copolymers such aspolystyrene butadiene and polystyrene/acrylate.

Examples of additional components which may be present, for example inan adhesive composition may include, but are not limited to, acrylics,styrene-butadiene copolymers, styrene-butadiene-styrene blockcopolymers, styrene-isoprene-styrene block copolymers, polyvinylbutyral,cellulose acetate butyrate, polyvinylpyrrolidone, polyurethanes,polyamides, ethylene-vinylacetate copolymers, epoxides, multifunctionalacrylates, vinyls, vinylethers, and their oligomers, polymers andcopolymers. Adhesive layers may also contain polyurethane dispersionsand water soluble polymer selected from the group consisting ofpolyvinyl alcohol; polyethylene glycol and its copolymers withpolypropylene glycol; poly(vinylpyrolidone) and its copolymers;polysaccharides; gelatin; poly(acrylic acid), its salt forms, and itscopolymers; poly(methacrylic acid), its salt forms, and its copolymers;poly(2-di methylaminoethyl methacrylate); poly(2-ethyl-2-oxazoline);poly(2-vinylpyridine); poly(allylamine); polyacrylamide;polymethacrylamide; and a cationic polymer functionalized withquaternary ammonium groups. Adhesive layers may be post cured by, forexample, heat or radiation such as UV after lamination.

The entirety of the stack, e.g., including a substrate (53), can besealed with an edge seal (51) as shown in FIG. 6. The edge seal (51) mayinclude any of the sealing compositions described above. The edge seal(51) may be continuous around the light-collimating layer (10) andsubstrate (53), or the edge seal (51) may only cover a portion of thestack, e.g., only the outer edge of the light-collimating layer (10). Insome embodiments, the edge seal (51) may include an additionalprotective layer, e.g., a layer that is impermeable to water, e.g.,clear polyethylene. The protective layer may provide moisture or gasbarrier properties. The edge of the protective layer and or edge sealmay be sealed with a thermal or UV curable or thermal activated edgeseal material that provides moisture or gas barrier properties. In anembodiment, the edge seal is sandwiched by two protective substrates.

In some embodiments, the edge seal (51) will actually incase the entirestack, thereby creating a sealed assembly. While not shown, it isunderstood that one or more electrical connections may have to traversethe edge seal (51) to provide an electrical connection to the first (12)and second (14) electrodes. Such connections may be provided by aflexible ribbon connector.

In addition to showing the details of the sealing layer (28), FIG. 6also illustrates how a light-collimating layer (10) can be laminatedonto the substrate (53) such as glass or another clear durable material.While it is not shown in FIG. 6, it is noted that the light-collimatinglayer (10) may be protected on both top and bottom with a substrate. Thetwo substrates may be different or the same, for example, a firstsubstrate may be glass and a second substrate may be polyethylene. Anedge seal (51) may extend around both top and bottom substrates and thelight-collimating layer (10) between substrates. Typically, an opticaladhesive (52), such as available from Delo Adhesives, is used to bondthe light-collimating layer (10) to the substrate(s) (53).Alternatively, a light-collimating layer (10) may be coated with acombination of an optical adhesive (52) and a release sheet (54) wherebythe light-collimating layer (10) with release sheet (54) can berolled-up and transported to an assembly facility where it will be cutto size. Prior to being deployed, the release sheet (54) can be removed,and the light-collimating layer (10) can be attached directly to thesubstrate (53), as illustrated in FIG. 7. The substrate may be any clearsurface for which light collimation is desired, such as a conferenceroom window, automotive glass, or a diffuser in an LCD stack.

Fabricating Light-Collimating Layer

A light collimating film can be produced using a roll-to-roll process asillustrated in FIG. 8, and described in detail in U.S. Pat. No.9,081,250. As shown in FIG. 8, the process involves a number of steps:In the first step a layer (60) of an embossing composition, e.g., athermoplastic, thermoset, or a precursor thereof, optionally with asolvent, is deposited on a conductive transparent film (61), such as afilm of polyethylene terephthalate (PET) including a layer of indium-tinoxide (PET-ITO). (The solvent, if present, readily evaporates.) A primerlayer (i.e., an electrode protection layer) may be used to increase theadhesion between the layer of embossing composition and the supportinglayer, which may be the PET. Additionally, an adhesion promoter may beused in the primer layer to improve adhesion to the supporting layer. Inthe second step, the layer (60) is embossed at a temperature higher thanthe glass transition temperature of the layer material by apre-patterned embossing tool (62), the fabrication of which is describedbelow. (The primer and/or adhesion promoter may be adjusted to decreaseadhesion to the embossing tool (62).) In the third step, the patternedlayer (60) is released from the embossing tool (62) preferably during orafter it is hardened, e.g., by cooling. The characteristic pattern ofthe elongated chambers (as described above) is now established. In stepfour, the elongated chambers (63) are filled with a bistableelectrophoretic fluid (64), described above. In some embodiments, thebistable electrophoretic fluid will include a sealing composition thatis incompatible with the electrophoretic fluid (64) and has a lowerspecific gravity than the solvent and the pigment particles in theelectrophoretic fluid (64). In such embodiments, the sealing compositionwill rise to the top of the elongated chambers (63), whereby it can behardened in subsequent steps. As an alternative (not shown in FIG. 8),the sealing composition may be overcoated after the elongated chambers(63) are filled with the electrophoretic fluid (64). In the next step,the elongated chambers (63) filled with electrophoretic fluid (64) aresealed by hardening the sealing composition, for example with UVradiation (65), or by heat, or moisture. In the sixth step, the sealedelongated chambers are laminated to a second transparent conductive film(66), which may be pre-coated with an optically clear adhesive layer(67), which may be a pressure sensitive adhesive, a hot melt adhesive, aheat, moisture, or radiation curable adhesive. [Preferred materials forthe optically-clear adhesive include acrylics, styrene-butadienecopolymers, styrene-butadiene-styrene block copolymers,styrene-isoprene-styrene block copolymers, polyvinylbutyal, celluloseacetate butyrate, polyvinylpyrrolidone, polyurethanes, polyamides,ethylene-vinylacetate copolymers, epoxides, multifunctional acrylates,vinyls, vinylethers, and their oligomers, polymers, and copolymers.] Inthe final step the finished sheets of switchable light-collimating filmmay be cut, e.g., with a knife edge (69), or with a laser cutter. Insome embodiments, an eighth step, including laminating anotheroptically-clear adhesive and a release sheet may be performed on thefinished switchable light-collimating film so that the film can beshipped in section sheets or rolls and cut to size when it is to beused, e.g., for incorporation into a display, a window, or otherdevice/substrate.

The embossing tool (62) may be prepared by a photoresist processfollowed by either etching or electroplating. It is then coated with alayer of photoresist and exposed to UV. A mask is placed between the UVand the layer of photoresist. In some embodiments, the unexposed orexposed areas are then removed by washing them with an appropriateorganic solvent or aqueous solution. The remaining photoresist is driedand sputtered again with a thin layer of seed metal. The master is thenready for electroforming. A typical material used for electroforming isnickel cobalt. Alternatively, the master can be made of nickel by nickelsulfamate electroforming or electroless nickel deposition. The floor ofthe embossing tool is typically between 50 and 5000 microns thick. Themaster can also be made using other microengineering techniquesincluding e-beam writing, dry etching, chemical etching, laser writingor laser interference as described in “Replication techniques formicro-optics”, SPIE Proc. Vol. 3099, pp 76-82 (1997). Alternatively, theembossing tool can be made by photomachining using plastics, ceramics ormetals. Several methods for embossing tool production are described ingreater detail below.

FIGS. 9A and 9B illustrate the embossing process with an embossing tool(111), with a three-dimensional microstructure (circled) on its surface.As shown in FIGS. 9A and 9B, after the embossing tool (111) is appliedto the embossing composition (112) of at least 20 μm thick, e.g., atleast 40 μm thick, e.g., at least 50 μm thick, e.g., at least 60 μmthick, e.g., at least 80 μm thick, e.g., at least 100 μm thick, e.g., atleast 150 μm, e.g., at least 200 μm thick, e.g., at least 250 μm thick.After the embossing composition is cured (e.g., by radiation), or thehot-embossable material becomes embossed by heat and pressure, theembossed material is released from the embossing tool (see FIG. 9B),leaving behind elongated chambers of the requisite dimensions, e.g.,wherein a height of the elongated chambers is equal to or less than thethickness of the collimating layer (embossing composition), and whereina width of the elongated chambers is between 9 μm and 150 μm, and alength of the chambers is between 200 μm and 5 mm.

Using a conventional embossing tool, the cured or hot embossed materialsometimes does not completely release from the tool because of theundesired strong adhesion between cured or hot embossed material and thesurface of the embossing tool. In this case, there may be some cured orhot embossed material transferred to, or stuck on, the surface of theembossing tool, leaving an uneven surface on the object formed from theprocess.

This problem is even more pronounced if the object is formed on asupporting layer, such as a transparent conductive layer or a polymericlayer. If the adhesion between the cured or hot embossed material andthe supporting layer is weaker than the adhesion between the cured orhot embossed material and the surface of the embossing tool, the releaseprocess of the cured or hot embossed material from the embossing toolmay cause separation of the object from the supporting layer.

In some cases, an object may be formed on a stack of layers, and in thiscase, if the adhesion between any two of the adjacent layers is weakerthan the adhesion between the cured or hot embossed material and thesurface of the embossing tool, the release process of the cured or hotembossed material from the embossing tool could cause a break-downbetween the two layers.

The above described problems are especially a concern when the curedembossing composition or hot embossed material does not adhere well tocertain supporting layers. For example, if the supporting layer is apolymeric layer, the adhesion between the polymeric layer and a cured orhot embossed embossing composition is weak in case one of them ishydrophilic and the other is hydrophobic. Therefore it is preferred thateither both of the embossing composition and the supporting layer arehydrophobic or both are hydrophilic.

Suitable hydrophilic compositions for forming the embossing layer orsupporting layer may comprise a polar oligomeric or polymeric material.As described in U.S. Pat. No. 7,880,958, such a polar oligomeric orpolymeric material may be selected from the group consisting ofoligomers or polymers having at least one of the groups such as nitro(—NO₂), hydroxyl (—OH), carboxyl (—COO), alkoxy (—OR wherein R is analkyl group), halo (e.g., fluoro, chloro, bromo or iodo), cyano (—CN),sulfonate (—SO₃) and the like. The glass transition temperature of thepolar polymer material is preferably below about 100° C. and morepreferably below about 60° C. Specific examples of suitable polaroligomeric or polymeric materials may include, but are not limited to,polyvinyl alcohol, polyacrylic acid, poly(2-hydroxylethyl methacrylate),polyhydroxy functionalized polyester acrylates (such as BDE 1025, BomarSpecialties Co, Winsted, Conn.) or alkoxylated acrylates, such asethoxylated nonyl phenol acrylate (e.g., SR504, Sartomer Company),ethoxylated trimethylolpropane triacrylate (e.g., SR9035, SartomerCompany) or ethoxylated pentaerythritol tetraacrylate (e.g., SR494, fromSartomer Company).

The embossing tool (111) may be used directly to emboss the composition(112). More typically, the embossing tool (111) is mounted on a plaindrum to allow rotation of the embossing sleeve over the embossingcomposition (112). The embossing drum or sleeve (121) is usually formedof a conductive material, such as a metal (e.g., aluminum, copper, zinc,nickel, chromium, iron, titanium, cobalt or the like), an alloy derivedfrom any of the aforementioned metals, or stainless steel. Differentmaterials may be used to form a drum or sleeve. For example, the centerof the drum or sleeve may be formed of stainless steel and a nickellayer is sandwiched between the stainless steel and the outermost layerwhich may be a copper layer.

Method A:

The embossing drum or sleeve (121) may be formed of a non-conductivematerial with a conductive coating or a conductive seed layer on itsouter surface, as shown in FIG. 10. Before coating, a photosensitivematerial (122) on the outer surface of a drum or sleeve (21), as shownin step B of FIG. 10, precision grinding and polishing may be used toensure smoothness of the outer surface of the drum or sleeve. Aphotosensitive material (122), e.g., a photoresist, can then be coatedon the outer surface of the drum or sleeve (121). The photosensitivematerial may be of a positive tone, negative tone or dual tone. Thephotosensitive material may also be a chemically amplified photoresist.The coating may be carried out using dip, spray or ring coating. Afterdrying and/or baking, the photosensitive material may be subjected toexposure, as shown in step C of FIG. 10, e.g., by exposing thephotosensitive material to a light source. Alternatively, thephotosensitive material (122) can be a dry film photoresist that islaminated onto the outer surface of the drum or sleeve (121). When a dryfilm is used, it is also exposed to a light source as described.

In step C of FIG. 10, a suitable light source (123), e.g., IR, UV,e-beam or laser, is used to expose the photosensitive material coated ora dry film photoresist (122) laminated on the drum or sleeve (121). Thelight source can be a continuous or pulsed light. A photomask (124) isoptionally used to define the three-dimensional microstructure to beformed. Depending on the microstructure, the exposure can bestep-by-step, continuous or a combination thereof. After exposure, thephotosensitive material (122) may be subjected to post-exposuretreatment, e.g., baking, before development. Depending on the tone ofthe photosensitive material, either exposed or un-exposed areas will beremoved by using a developer. After development, the drum or sleeve witha patterned photosensitive material (125) on its outer surface (as shownin Step D of FIG. 10) may be subjected to baking or blanket exposurebefore deposition (e.g., electroplating, electroless plating, physicalvapor deposition, chemical vapor deposition or sputtering deposition).The thickness of the patterned photosensitive material is preferablygreater than the depth or height of the three-dimensional microstructureto be formed.

A metal or alloy (e.g., nickel, cobalt, chrome, copper, zinc or an alloyderived from any of the aforementioned metals) can be electroplatedand/or electroless plated onto the drum or sleeve. The plating material(126) is deposited on the outer surface of the drum or sleeve in areasthat are not covered by the patterned photosensitive material. Thedeposit thickness is preferably less than that of the photosensitivematerial, as shown in step E of FIG. 10. The thickness variation of thedeposit over the whole drum or sleeve area can be controlled to be lessthan 1%, by adjusting plating conditions, e.g., the distance between theanode and the cathode (i.e., drum or sleeve) if electroplating is used,the rotation speed of the drum or sleeve and/or circulation of theplating solution.

Alternatively, in the case of using electroplating to deposit theplating material (126), the thickness variation of the deposit over theentire surface of the drum or sleeve may be controlled by inserting anon-conductive thickness uniformer between a cathode (i.e., the drum orsleeve) and an anode, as described in U.S. Pat. No. 8,114,262, thecontent of which is incorporated herein by reference in its entirety.

After plating, the patterned photosensitive material (125) can bestripped by a stripper (e.g., an organic solvent or aqueous solution). Aprecision polishing may be optionally employed to ensure acceptablethickness variation and degree of roughness of the deposit (126) overthe entire drum or sleeve. Step F of FIG. 10 shows a cross-section viewof an embossing drum or sleeve with a three-dimensional patternmicrostructure formed thereon.

Method B:

Alternatively, a three-dimensional microstructure may be formed on aflat substrate, as shown in FIG. 11. In Step A of FIG. 11, aphotosensitive material (142) is coated on a substrate layer (141)(e.g., a glass substrate). The photosensitive material (142), as statedabove, may be of a positive tone, negative tone or dual tone. Thephotosensitive material (142) may also be a chemically amplifiedphotoresist. The coating may be carried out using dip, spray, slot die,or spin coating. After drying and/or baking, the photosensitive materialis subjected to exposure to a suitable light source (not shown) througha photomask (not shown). Alternatively, the photosensitive material(142) can be a dry film photoresist (which is usually commerciallyavailable) that is laminated onto the substrate (141). The dry film isalso exposed to a light source as described above.

In Step B of FIG. 11, after exposure, depending on the tone of thephotosensitive material, either the exposed or un-exposed areas of thephotosensitive material will be removed by using a developer. Afterdevelopment, the substrate layer (141) with the remaining photosensitivematerial (142) may be subjected to baking or blanket exposure beforeStep C. The thickness of the remaining photosensitive material should bethe same as the depth or height of the three-dimensional microstructureto be formed. In the Step C, an electrical conductive seed layer (143)is coated over the remaining photosensitive material (142) and thesubstrate (141) in areas not occupied by the photosensitive material.The electrical conductive seed layer is usually formed of silver,however other conductive materials, such as gold or nickel may also beused.

In Step D, a metal or alloy (144) (e.g., nickel, cobalt, chrome, copper,zinc, or an alloy derived from any of the aforementioned metals) iselectroplated and/or electroless plated onto the surface covered byelectrical conductive seed layer and the plating process is carried outuntil there is enough plated material thickness (h) over the patternedphotosensitive material. The thickness (h) in Step D of FIG. 11 ispreferably 25 to 5000 microns, and more preferably 25 to 1000 microns.

After plating, the plated material (144) is separated from the substratelayer (141) which is peeled off. The photosensitive material (142) alongwith the electrical conductive seed layer (143) is removed. Thephotosensitive material may be removed by a stripper (e.g., an organicsolvent or aqueous solution). The electrical conductive seed layer (143)may be removed by an acidic solution (e.g., sulfuric/nitric mixture) orcommercially available chemical strippers, leaving behind only a metalsheet (144) having a three-dimensional structure on one side and beingflat on the other side. A precision polishing may be applied to themetal sheet (144), after which the flat shim may be used directly forembossing, or it may be mounted on (i.e., wrapped over) a drum with thethree-dimensional microstructure on the outer surface to form anembossing tool. A precious metal or alloy thereof is finally coated overthe entire surface of the embossing tool, as described above. As statedabove, gold or its alloy is preferred over other precious metals andalloys due to its lack of reactivity.

Method C:

A further alternative method is demonstrated in FIG. 12. This method issimilar to that of FIG. 11, but simplified. Instead of an electricalconductive seed layer such as silver, a layer of precious metal or alloythereof (153) is simply coated over the photosensitive material (152).As stated above, gold or its alloy is preferred. Consequently, in StepE, after the plated material (154) is separated from the substrate(151), only the photosensitive material (152) is removed, the gold oralloy coating (153) remains with the metal sheet (154) with athree-dimensional structure on one side and being flat on the otherside.

Examples of components in a composition for forming the collimatinglayer, may include, but are not limited to, thermoplastic or thermosetmaterials or a precursor thereof, such as multifunctional vinylsincluding, but not limited to, acrylates, methacrylates, allyls,vinylbenzenes, vinylethers, multifunctional epoxides and oligomers orpolymers thereof, and the like. Multifunctional acrylate and oligomersthereof are often used. A combination of a multifunctional epoxide and amultifunctional acrylate is also useful to achieve desirablephysico-mechanical properties of the collimating layer. A low Tg (glasstransition temperature) binder or crosslinkable oligomer impartingflexibility, such as urethane acrylate or polyester acrylate, may alsobe added to improve the flexure resistance of the embossed privacylayers.

Further examples of compositions for a collimating layer may comprise apolar oligomeric or polymeric material. Such a polar oligomeric orpolymeric material may be selected from the group consisting ofoligomers or polymers having at least one of the groups such as nitro(—NO₂), hydroxyl (—OH), carboxyl (—COO), alkoxy (—OR wherein R is analkyl group), halo (e.g., fluoro, chloro, bromo or iodo), cyano (—CN),sulfonate (—SO₃) and the like. The glass transition temperature of thepolar polymer material is preferably below about 100° C., and morepreferably below about 60° C. Specific examples of suitable polaroligomeric or polymeric materials may include, but are not limited to,polyhydroxy functionalized polyester acrylates (such as BDE 1025, BomarSpecialties Co, Winsted, Conn.) or alkoxylated acrylates, such asethoxylated nonyl phenol acrylate (e.g., SR504, Sartomer Company),ethoxylated trimethylolpropane triacrylate (e.g., SR9035, SartomerCompany) or ethoxylated pentaerythritol tetraacrylate (e.g., SR494, fromSartomer Company).

Alternatively, the collimating layer composition may comprise (a) atleast one difunctional UV curable component, (b) at least onephotoinitiator, and (c) at least one mold release agent. Suitabledifunctional components may have a molecular weight higher than about200. Difunctional acrylates are preferred and difunctional acrylateshaving a urethane or an ethoxylated backbone are particularly preferred.More specifically, suitable difunctional components may include, but arenot limited to, diethylene glycol diacrylate (e.g., SR230 fromSartomer), triethylene glycol diacrylate (e.g., SR272 from Sartomer),tetraethylene glycol diacrylate (e.g., SR268 from Sartomer),polyethylene glycol diacrylate (e.g., SR295, SR344 or SR610 fromSartomer), polyethylene glycol dimethacrylate (e.g., SR603, SR644, SR252or SR740 from Sartomer), ethoxylated bisphenol A diacrylate (e.g.,CD9038, SR349, SR601 or SR602 from Sartomer), ethoxylated bisphenol Adimethacrylate (e.g., CD540, CD542, SR101, SR150, SR348, SR480 or SR541from Sartomer), and urethane diacrylate (e.g., CN959, CN961, CN964,CN965, CN980 or CN981 from Sartomer; Ebecryl 230, Ebecryl 270, Ebecryl8402, Ebecryl 8804, Ebecryl 8807 or Ebecryl 8808 from Cytec). Suitablephotoinitiators may include, but are not limited to, bis-acyl-phosphineoxide,2-benzyl-2-(dimethylamino)-1-[4-(4-morpholinyl)phenyl]-1-butanone,2,4,6-trimethylbenzoyl diphenyl phosphine oxide,2-isopropyl-9H-thioxanthen-9-one, 4-benzoyl-4′-methyldiphenylsulphideand 1-hydroxy-cyclohexyl-phenyl-ketone,2-hydroxy-2-methyl-1-phenyl-propan-1-one,1-[4-(2-hydroxyethoxy)-phenyl]-2-hydroxy-2-methyl-1-propane-1-one,2,2-dimethoxy-1,2-diphenylethan-1-one or2-methyl-1[4-(methylthio)phenyl]-2-morpholinopropan-1-one. Suitable moldrelease agents may include, but are not limited to, organomodifiedsilicone copolymers such as silicone acrylates (e.g., Ebercryl 1360 orEbercyl 350 from Cytec), silicone polyethers (e.g., Silwet 7200, Silwet7210, Silwet 7220, Silwet 7230, Silwet 7500, Silwet 7600 or Silwet 7607from Momentive). The composition may further optionally comprise one ormore of the following components, a co-initiator, monofunctional UVcurable component, multifunctional UV curable component or stabilizer.

Two geometries of light-collimating layers resulting from thefabrication methods described above a shown in FIGS. 13 and 14 (viewfrom above). These geometries show the general trend in the aspectratios of the elongated chambers (22) in that they are longer in onedirection (L) than another (W). That is, the length (L) of an elongatedchamber is typically at least twice the width (W) of the elongatedchamber, e.g., at least three times the width of the elongated chamber,e.g., at least four times the width of the elongated chamber, e.g., atleast five times the width of the elongated chamber, e.g., at least tentimes the width of the elongated chamber. [As discussed above, theheight (H) (out of the plane of the page in FIGS. 13 and 14) of theelongated chambers is equal to or less than the thickness of thecollimating layer.] Typically, the width of each elongated chamber isbetween 9 μm and 150 μm. Typically, the length of each elongated chamberis between 200 μm and 5 mm.

The spacing between rows (A) (a.k.a. “pitch”) plays a major role indetermining how much the viewing angle is reduced when theelectrophoretic pigments (26) are fully distributed in the elongatedchambers (22), as discussed previously. If the height of the elongatedchambers (22) remains constant, the viewing angle narrows withdecreasing spacing “A.” However, decreasing “A” means that there is morebistable electrophoretic fluid (24) with pigment particles for the lightto traverse, and the overall light transmission of the light-collimatingfilm decreases. In a similar fashion, the gap width “G” between adjacentelongated chambers within the same row also affects the overalltransmission of the light-collimating layer because of the amount ofscattering particles between the light source and the viewer. Thus, theoverall transmission of FIG. 13 is lower than the overall transmissionof FIG. 14. However, there is less “leakage” of non-collimated light inFIG. 13 because there are fewer off-axis pathways for incident light totravel past the elongated chambers.

In some embodiments, when the elongated chambers are created with arolling embossing tool, e.g., as described above, the elongated chambersare formed in rows and columns (as viewed from above). In an effort tominimize leakage the gaps between adjacent elongated chambers in thefirst row are offset horizontally from the gaps between adjacentelongated chambers in the second row in both FIG. 13 and FIG. 14. Ingeneral the gap width “G” between adjacent elongated chambers within thesame row is less than 30 μm, e.g., less than 25 μm, e.g., less than 20μm, e.g., less than 15 μm, e.g., less than 10 μm. The gaps betweenadjacent elongated chambers in successive rows may be offset by at least1 μm, e.g., at least 2 μm, e.g., at least 3 μm, e.g., at least 5 μm. Insome embodiments, the entire gap of a first row is spanned by theelongated chamber of a second row, as shown in FIG. 14. In mostembodiments, L>G. In many embodiments, L>>G. In most embodiments A>W. Inmany embodiments A>>W.

Numerous patents and applications assigned to or in the names of theMassachusetts Institute of Technology (MIT), E Ink Corporation, E InkCalifornia, LLC, and related companies describe various technologiesused in encapsulated and microcell electrophoretic and otherelectro-optic media. Encapsulated electrophoretic media comprisenumerous small capsules, each of which itself comprises an internalphase containing electrophoretically-mobile particles in a fluid medium,and a capsule wall surrounding the internal phase. Typically, thecapsules are themselves held within a polymeric binder to form acoherent layer positioned between two electrodes. In a microcellelectrophoretic display, the charged particles and the fluid are notencapsulated within microcapsules but instead are retained within aplurality of cavities formed within a carrier medium, typically apolymeric film.

The technologies described in these patents and applications include:(a)

Electrophoretic particles, fluids and fluid additives; see for exampleU.S. Pat. Nos. 7,002,728 and 7,679,814; as well as U.S. PatentApplications Publication No. 2016/0170106; (b) Capsules, binders andencapsulation processes; see for example U.S. Pat. Nos. 6,922,276 and7,411,719; as well as U.S. Patent Applications Publication No.2011/0286081; (c) Microcell structures, wall materials, and methods offorming microcells; see for example U.S. Pat. Nos. 6,672,921; 6,751,007;6,753,067; 6,781,745; 6,788,452; 6,795,229; 6,806,995; 6,829,078;6,833,177; 6,850,355; 6,865,012; 6,870,662; 6,885,495; 6,906,779;6,930,818; 6,933,098; 6,947,202; 6,987,605; 7,046,228; 7,072,095;7,079,303; 7,141,279; 7,156,945; 7,205,355; 7,233,429; 7,261,920;7,271,947; 7,304,780; 7,307,778; 7,327,346; 7,347,957; 7,470,386;7,504,050; 7,580,180; 7,715,087; 7,767,126; 7,880,958; 8,002,948;8,154,790; 8,169,690; 8,441,432; 8,582,197; 8,891,156; 9,279,906;9,291,872; and 9,388,307; and U.S. Patent Applications Publication Nos.2003/0175480; 2003/0175481; 2003/0179437; 2003/0203101; 2013/0321744;2014/0050814; 2015/0085345; 2016/0059442; 2016/0004136; and2016/0059617; (d) Methods for filling and sealing microcells; see forexample U.S. Pat. Nos. 6,545,797; 6,751,008; 6,788,449; 6,831,770;6,833,943; 6,859,302; 6,867,898; 6,914,714; 6,972,893; 7,005,468;7,046,228; 7,052,571; 7,144,942; 7,166,182; 7,374,634; 7,385,751;7,408,696; 7,522,332; 7,557,981; 7,560,004; 7,564,614; 7,572,491;7,616,374; 7,684,108; 7,715,087; 7,715,088; 8,179,589; 8,361,356;8,520,292; 8,625,188; 8,830,561; 9,081,250; and 9,346,987; and U.S.Patent Applications Publication Nos. 2002/0188053; 2004/0120024;2004/0219306; 2006/0132897; 2006/0164715; 2006/0238489; 2007/0035497;2007/0036919; 2007/0243332; 2015/0098124; and 2016/0109780; (e) Filmsand sub-assemblies containing electro-optic materials; see for exampleU.S. Pat. Nos. 6,825,829; 6,982,178; 7,112,114; 7,158,282; 7,236,292;7,443,571; 7,513,813; 7,561,324; 7,636,191; 7,649,666; 7,728,811;7,729,039; 7,791,782; 7,839,564; 7,843,621; 7,843,624; 8,034,209;8,068,272; 8,077,381; 8,177,942; 8,390,301; 8,482,835; 8,786,929;8,830,553; 8,854,721; 9,075,280; and 9,238,340; and U.S. PatentApplications Publication Nos. 2007/0237962; 2009/0109519; 2009/0168067;2011/0164301; 2014/0115884; and 2014/0340738; (f) Backplanes, adhesivelayers and other auxiliary layers and methods used in displays; see forexample U.S. Pat. Nos. 7,116,318; 7,535,624; and 9,310,661; as well asU.S. Patent Applications Publication Nos. 2016/0103380; and2016/0187759; (g) Methods for driving displays; see for example U.S.Pat. Nos. 5,930,026; 6,445,489; 6,504,524; 6,512,354; 6,531,997;6,753,999; 6,825,970; 6,900,851; 6,995,550; 7,012,600; 7,023,420;7,034,783; 7,061,166; 7,061,662; 7,116,466; 7,119,772; 7,177,066;7,193,625; 7,202,847; 7,242,514; 7,259,744; 7,304,787; 7,312,794;7,327,511; 7,408,699; 7,453,445; 7,492,339; 7,528,822; 7,545,358;7,583,251; 7,602,374; 7,612,760; 7,679,599; 7,679,813; 7,683,606;7,688,297; 7,729,039; 7,733,311; 7,733,335; 7,787,169; 7,859,742;7,952,557; 7,956,841; 7,982,479; 7,999,787; 8,077,141; 8,125,501;8,139,050; 8,174,490; 8,243,013; 8,274,472; 8,289,250; 8,300,006;8,305,341; 8,314,784; 8,373,649; 8,384,658; 8,456,414; 8,462,102;8,514,168; 8,537,105; 8,558,783; 8,558,785; 8,558,786; 8,558,855;8,576,164; 8,576,259; 8,593,396; 8,605,032; 8,643,595; 8,665,206;8,681,191; 8,730,153; 8,810,525; 8,928,562; 8,928,641; 8,976,444;9,013,394; 9,019,197; 9,019,198; 9,019,318; 9,082,352; 9,171,508;9,218,773; 9,224,338; 9,224,342; 9,224,344; 9,230,492; 9,251,736;9,262,973; 9,269,311; 9,299,294; 9,373,289; 9,390,066; 9,390,661; and9,412,314; and U.S. Patent Applications Publication Nos. 2003/0102858;2004/0246562; 2005/0253777; 2007/0091418; 2007/0103427; 2007/0176912;2008/0024429; 2008/0024482; 2008/0136774; 2008/0291129; 2008/0303780;2009/0174651; 2009/0195568; 2009/0322721; 2010/0194733; 2010/0194789;2010/0220121; 2010/0265561; 2010/0283804; 2011/0063314; 2011/0175875;2011/0193840; 2011/0193841; 2011/0199671; 2011/0221740; 2012/0001957;2012/0098740; 2013/0063333; 2013/0194250; 2013/0249782; 2013/0321278;2014/0009817; 2014/0085355; 2014/0204012; 2014/0218277; 2014/0240210;2014/0240373; 2014/0253425; 2014/0292830; 2014/0293398; 2014/0333685;2014/0340734; 2015/0070744; 2015/0097877; 2015/0109283; 2015/0213749;2015/0213765; 2015/0221257; 2015/0262255; 2015/0262551; 2016/0071465;2016/0078820; 2016/0093253; 2016/0140910; and 2016/0180777.

The manufacture of a three-layer electro-optic display normally involvesat least one lamination operation. For example, in several of theaforementioned MIT and E Ink patents and applications, there isdescribed a process for manufacturing an encapsulated electrophoreticdisplay in which an encapsulated electrophoretic medium comprisingcapsules in a binder is coated on to a flexible substrate comprisingindium-tin-oxide (ITO) or a similar conductive coating (which acts asone electrode of the final display) on a plastic film, thecapsules/binder coating being dried to form a coherent layer of theelectrophoretic medium firmly adhered to the substrate. Separately, abackplane, containing an array of pixel electrodes and an appropriatearrangement of conductors to connect the pixel electrodes to drivecircuitry, is prepared. To form the final display, the substrate havingthe capsule/binder layer thereon is laminated to the backplane using alamination adhesive. In one embodiment, the backplane is itself flexibleand is prepared by printing the pixel electrodes and conductors on aplastic film or other flexible substrate. In other embodiments bothelectrodes are flexible, thereby allowing the constructedelectrophoretic display to be flexible. The obvious lamination techniquefor mass production of displays by this process is roll lamination usinga lamination adhesive. Similar manufacturing techniques can be used withother types of electro-optic displays. For example, a microcellelectrophoretic medium may be laminated to a backplane or flexibleelectrode in substantially the same manner as an encapsulatedelectrophoretic medium.

U.S. Pat. No. 6,982,178 describes a method of assembling a solidelectro-optic display (including an encapsulated electrophoreticdisplay) which is well adapted for mass production. Essentially, thispatent describes a so-called “front plane laminate” (“FPL”) whichcomprises, in order, a light-transmissive electrically-conductive layer;a layer of a solid electro-optic medium in electrical contact with theelectrically-conductive layer; an adhesive layer; and a release sheet.Typically, the light-transmissive electrically-conductive layer will becarried on a light-transmissive substrate, which is preferably flexible,in the sense that the substrate can be manually wrapped around a drum(say) 10 inches (254 mm) in diameter without permanent deformation. Theterm “light-transmissive” is used in this patent and herein to mean thatthe layer thus designated transmits sufficient light to enable anobserver, looking through that layer, to observe the change in displaystates of the electro-optic medium, which will normally be viewedthrough the electrically-conductive layer and adjacent substrate (ifpresent); in cases where the electro-optic medium displays a change inreflectivity at non-visible wavelengths, the term “light-transmissive”should of course be interpreted to refer to transmission of the relevantnon-visible wavelengths. The substrate will typically be a polymericfilm, and will normally have a thickness in the range of about 1 toabout 25 mil (25 to 634 μm), preferably about 2 to about 10 mil (51 to254 μm). The electrically-conductive layer is conveniently a thin metalor metal oxide layer of, for example, aluminum or ITO, or may be aconductive polymer. Poly(ethylene terephthalate) (PET) films coated withaluminum or ITO are available commercially, for example as “aluminizedMylar” (“Mylar” is a Registered Trade Mark) from E.I. du Pont de Nemours& Company, Wilmington Del., and such commercial materials may be usedwith good results in the front plane laminate.

Assembly of an electrophoretic display using such a front plane laminatemay be effected by removing the release sheet from the front planelaminate and contacting the adhesive layer with the backplane underconditions effective to cause the adhesive layer to adhere to thebackplane, thereby securing the adhesive layer, layer of electrophoreticmedium and electrically-conductive layer to the backplane. This processis well-adapted to mass production since the front plane laminate may bemass produced, typically using roll-to-roll coating techniques, and thencut into pieces of any size needed for use with specific backplanes.

The term “impulse” is used herein in its conventional meaning of theintegral of voltage with respect to time. However, some bistableelectrophoretic media act as charge transducers, and with such media analternative definition of impulse, namely the integral of current overtime (which is equal to the total charge applied) may be used. Theappropriate definition of impulse should be used, depending on whetherthe medium acts as a voltage-time impulse transducer or a charge impulsetransducer.

A further complication in driving electrophoretic displays is the needfor so-called “DC balance”. U.S. Pat. Nos. 6,531,997 and 6,504,524discuss problems may be encountered, and the working lifetime of adisplay reduced, if the method used to drive the display does not resultin zero, or near zero, net time-averaged applied electric field acrossthe electrophoretic medium. A drive method which does result in zero nettime-averaged applied electric field across the electrophoretic mediumis conveniently referred to a “direct current balanced” or “DCbalanced”.

As already noted, an encapsulated electrophoretic medium typicallycomprises electrophoretic capsules disposed in a polymeric binder, whichserves to form the discrete capsules into a coherent layer. Thecontinuous phase in a polymer-dispersed electrophoretic medium, and thecell walls of a microcell medium serve similar functions. It has beenfound by E Ink researchers that the specific material used as the binderin an electrophoretic medium can affect the electro-optic properties ofthe medium. Among the electro-optic properties of an electrophoreticmedium affected by the choice of binder is the so-called “dwell timedependence”. As discussed in the U.S. Pat. No. 7,119,772 (see especiallyFIG. 34 and the related description), in some cases, the impulsenecessary for a transition between two specific optical states of abistable electrophoretic display varies with the residence time of apixel in its initial optical state, and this phenomenon is referred toas “dwell time dependence” or “DTD”. Obviously, it is desirable to keepDTD as small as possible since DTD affects the difficulty of driving thedisplay and may affect the quality of the image produced; for example,DTD may cause pixels which are supposed to form an area of uniform graycolor to differ slightly from one another in gray level, and the humaneye is very sensitive to such variations. Although it has been knownthat the choice of binder affects DTD, choosing an appropriate binderfor any specific electrophoretic medium has hitherto been based ontrial-and-error, with essentially no understanding of the relationshipbetween DTD and the chemical nature of the binder.

U.S. Patent Application Publication No. 2005/0107564 describes anaqueous polyurethane dispersion comprising a polyurethane polymercomprising the reaction product of: (a) an isocyanate terminatedprepolymer comprising the reaction product of (i) at least onepolyisocyanate comprising a,a,a,a-tetramethylxylene diisocyanate[systematic name 1.3-bis(1-isocyanato-1-methylethyl)benzene; thismaterial may hereinafter be called “TMXDI”]; (ii) at least onedifunctional polyol comprising polypropylene glycol, and (iii) anisocyanate reactive compound comprising an acid functional group and atleast two isocyanate reactive groups selected from a hydroxy, a primaryamino, a secondary amino, and combinations thereof; (b) a neutralizingagent comprising a tertiary amino group; (c) a monofunctional chainterminating agent; (d) a chain extending agent comprising an organicdiamine; and (e) water. This polyurethane dispersion, which mayhereinafter be called the “TMXDI/PPO” dispersion, has been found to beuseful as a lamination adhesive in electrophoretic displays.

From the foregoing, it will be seen that the present invention canprovide a switchable light-collimating film and devices that incorporateswitchable light-collimating films. In particular, the inventionprovides light-collimating films that are bistable and able to maintainwide and narrow viewing conditions with no additional energy input.

It will be apparent to those skilled in the art that numerous changesand modifications can be made in the specific embodiments of theinvention described above without departing from the scope of theinvention. Accordingly, the whole of the foregoing description is to beinterpreted in an illustrative and not in a limitative sense.

1. A method for driving a switchable light-collimating film, wherein theswitchable light-collimating film includes: a first light-transmissiveelectrode layer, a collimating layer having a thickness of at least 20μm, and comprising a plurality of elongated chambers, each elongatedchamber having an opening, a bistable electrophoretic fluid comprisingpigment particles disposed in each elongated chamber, a sealing layerthat seals the bistable electrophoretic fluid within at least one of theplurality of elongated chambers by spanning the opening of the elongatedchamber, and a second light-transmissive electrode layer, wherein thefirst and second light transmissive layers are disposed on either sideof the collimating layer, the method comprising applying a time varyingvoltage between the first and the second light-transmissive electrodelayers.
 2. The method of claim 1, wherein the collimating layer has athickness of less than 500 μm.
 3. The method of claim 1, wherein aheight of the elongated chambers is equal to or less than the thicknessof the collimating layer, a width of the elongated chambers is between 5μm and 150 μm, and a length of the chambers is between 200 μm and 5 mm.4. The method of claim 1, wherein the first or second light-transmissiveelectrode layer comprises indium-tin-oxide.
 5. The method of claim 1,wherein the time varying voltage includes a first voltage for a firsttime that has a first polarity and a second voltage for a second timethat has a second polarity.
 6. The method of claim 5, wherein the firstvoltage has a first magnitude, the second voltage has a secondmagnitude, and the first and second magnitudes are not equal.
 7. Themethod of claim 5, wherein the first voltage has a first magnitude, thesecond voltage has a second magnitude, and the first and secondmagnitudes are not equal.
 8. The method of claim 5, wherein the productof the first voltage and the first time is a first impulse, the productof the second voltage and the second time is a second impulse, and thefirst impulse and the second impulse are not equal in magnitude.
 9. Themethod of claim 5, wherein the product of the first voltage and thefirst time is a first impulse, the product of the second voltage and thesecond time is a second impulse, and the first impulse and the secondimpulse are equal in magnitude.
 10. The method of claim 5, wherein thetime varying voltage further includes the first voltage for a third timethat has the first polarity and the second voltage for a fourth timethat has the second polarity.
 11. The method of claim 1, wherein thebistable electrophoretic fluid comprises polymer-functionalized pigmentparticles and free polymer in a non-polar solvent.
 12. The method ofclaim 1, wherein the elongated chambers are arranged in rows and columnswhen the collimating layer is viewed from above, wherein the longerdimension of the elongated chambers run along rows, and wherein the rowsare separated from each other by at least three times the width of theelongated chambers.
 13. The method of claim 1, wherein the elongatedchambers are arranged in rows and columns when the collimating layer isviewed from above, and wherein the adjacent elongated chambers withinthe same row are separated by a gap of less than 30 μm.
 14. The methodof claim 1, wherein the time varying voltage is between 5 V and 150 V inmagnitude.
 15. The method of claim 14, wherein the time varying voltageis between 80 V and 120 V in magnitude.
 16. A display incorporating aswitchable light-collimating film using the method of claim
 1. 17. Awindow or door comprising a glass substrate and a switchablelight-collimating film using the method of claim
 1. 18. A displaycomprising: a light source; a switchable light-collimating filmcomprising: a first light-transmissive electrode layer, a collimatinglayer having a thickness of at least 20 μm, and comprising a pluralityof elongated chambers, each elongated chamber having an opening, abistable electrophoretic fluid comprising pigment particles disposed ineach elongated chamber, a sealing layer that seals the bistableelectrophoretic fluid within an elongated chamber by spanning theopening, and a second light-transmissive electrode layer, wherein thefirst and second light transmissive layers are disposed on either sideof the collimating layer; an active matrix of thin film transistors; aliquid crystal layer; a color filter array; a voltage source; and acontroller to provide a voltage impulse between the first and secondlight-transmissive electrode layers, wherein the voltage impulse causesa change in the effective transmission angle of the display.
 19. Thedisplay of claim 18, further comprising a prism film disposed betweenthe light source and the switchable light-collimating film.
 20. Thedisplay of claim 19, further comprising a diffusion layer between theprism film and the light source.
 21. The display of claim 18, whereinwhen the voltage source and controller provide a time varying impulse,the effective transmission angle of the display narrows.